Diesel combustion may generate emissions, including particulate matter (PM). The particulate matter may include diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, PM can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers. Various technologies have been developed for identifying and filtering out exhaust PMs before the exhaust is released to the atmosphere.
As an example, PM or soot sensors may be used in vehicles having internal combustion engines. A PM sensor may be located upstream and/or downstream of a diesel particulate filter (DPP), and may be used to sense PM loading on the filter and diagnose operation of the DPF. Resistive PM sensors may include interdigitated electrodes that sense a particulate matter or soot load based on a correlation between a measured change in electrical conductivity (or resistivity) between a pair of electrodes placed on a planar substrate surface of the sensor with the amount of PM deposited between the measuring electrodes. Specifically, the measured conductivity provides a measure of soot accumulation. However, resistive sensors may not deliver real-time measurement of soot, as there may be extended times while the sensor is just accumulating soot during which the sensor may not provide any real-time signal at all.
An example attempt to address this issue is shown in U.S. Pat. No. 8,713,991 where a real-time sensor, such as a high voltage PM sensor, is used. Therein, the high voltage PM sensor includes a single pair of electrodes and accumulates soot due to electrostatic capture similar to the resistive soot; however, in this case, soot begins to aggregate due to the strong electric field applied between the electrodes. When the soot aggregates break off the electrode, they carry part of the charge of the electrode with them and deposit their charge when they contact a grounded part of the sensor or the exhaust system. This charge deposition on the grounded part of the sensor may be detected as a current pulse flowing between the electrode and ground.
However, the inventors herein have recognized potential issues with such systems. As one example, the sensitivity of the high voltage sensor may change abruptly when there is any abrupt change in exhaust flow rate causing transients in the measured current. The flow of the soot aggregates towards the grounded part of the sensor may depend on the exhaust flow rate, for example. As such, an increased exhaust flow may result in a decrease in the amount of aggregates falling onto the grounded thereby causing a drop in the measured current. Likewise, a decreased exhaust flow rate may increase the amount of aggregates reaching the ground, thereby causing a spike in the measured current. Due to these current transients in the sensor output, the sensor capturing soot exiting the DPF may not truly reflect the DPF filtering capabilities.
In one example, the issues described above may be partially addressed by a method comprising collecting and charging particulate matter (PM) in an exhaust entering a PM sensor by applying a first voltage only to a first set of electrodes housed within the PM sensor, and measuring the charged PM by applying a second voltage only to a second set of electrodes housed within the PM sensor further separated from the first set of electrodes by a distance, the first voltage being higher than the second voltage. In this way, by juxtaposing two sets of electrodes within the same PM sensor housing, the second set of electrodes may be used for detecting the PMs in the exhaust. Aggregates exiting the first set of electrodes may be captured by the second set of electrodes, thereby reducing current transients in the second set of electrodes. Thus sensitivity to the sensor output on exhaust flow rate may be reduced and the sensor output may begin to more closely measure the DPF filtering capabilities more accurately and reliably in real-time.
As one example, by using the first set of electrodes primarily for charging the soot and the second set of electrodes primarily for measuring the highly charged soot, any transients in the measured current in the second set of electrodes may be reduced. In this way, by juxtaposing the second set of electrodes with the first set of electrodes, the charged soot exiting the first set of electrodes may be captured by the second set of electrodes thereby rendering the sensor more independent of the exhaust flow rate. Overall, these characteristics of the sensor may cause an output of the PM sensor to be more accurate, thereby increasing the accuracy of estimating particulate loading on a particulate filter.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.